This invention relates broadly to the catalytic oxidation of methanol with molecular oxygen to produce formaldehyde. More particularly it relates to those processes in which the catalytic oxidation of methanol is carried out by passing gases comprising methanol and molecular oxygen through two sequential catalyst beds, both of which comprise silver. In particular it relates to such a process in which the second catalyst bed contains particulate silver, especially silver crystals. In a particular embodiment, both catalyst beds consist essentially of silver crystals.
There is an extensive body of prior art dealing with the catalytic oxidation of methanol to form formaldehyde. Some processes, to which the present invention is not directed and which will therefore not be discussed further herein, employ catalysts comprising the oxides of any of a large number of catalytic elements, normally mixtures of the oxides of several elements. Typically such processes are conducted with an excess of oxygen and are characterized by very high conversions even in a single reaction stage. Others, to which the present process improvement is more closely related, employ two sequential reaction stages with the total amount of molecular oxygen being less than that required stoichiometrically to convert the methanol to formaldehyde, hydrogen, and water. Various catalysts are known for processes of this type, including silver, used either on an inert support or else as the metal itself in the form of gauze or crystals. Metallic foam is also a catalyst. Broadly speaking, it is known in these prior-art two-stage processes that such parameters as space velocity in the catalyst beds are significant in controlling the system to optimize process throughput and reaction efficiency. It is also known to introduce additional methanol, molecular oxygen, or both in between the two reaction stages although references to using supplemental methanol between stages where both stages use silver have not been seen. As will be seen hereinbelow, the present invention is directed to controlling two particular reaction parameters in a manner which has not been recognized in the prior art as being significant.
U.S. Pat. No. 3,959,383 (to Northeimer) discloses a two-stage reaction system in which methanol is reacted with molecular oxygen to produce formaldehyde. The first stage is carried out in the presence of a silver gauze catalyst although other forms of silver are not excluded, while the second stage is 20-30 mesh electrolytically-prepared silver crystals. Supplemental air is introduced between the two reaction stages, but supplemental methanol is not. The crux of the invention as disclosed and claimed is the maintenance of certain specified space velocities in the two reaction stages. The ratio of oxygen to methanol is taught as being significant, but there is no teaching that maintaining any particular ratio of formaldehyde to methanol in the feed to the second reaction stage is in any way significant. The incorporation of steam into the reaction mixture prior to the catalytic oxidation is mentioned. Over-all, the teaching of the Northeimer patent is directed primarily to the control of reaction space velocity and oxygen:methanol ratio. The concept of "specific reaction rate" as dealt with in the present invention does not appear in Northeimer.
The use of silver crystals as a methanol-oxidation catalyst is discussed in U.S. Pat. No. 1,968,552 (to Bond) and also in U.S. Pat. No. 1,937,381 (also to Bond). The manufacture of electrolytic silver crystals suitable for use as catalyst in those processes has been generally known in the art at least since the end of World War II, such crystals having been used in Germany during and since the war.
U.S. Pat. No. 2,462,413 (to Meath) discloses a two-stage reaction using a supported silver catalyst in each stage. Inter-stage cooling is employed, and additional reaction air is added between the two stages. U.S. Pat. No. 2,519,788 (to Payne) discloses a two-stage reaction in which the first-stage catalyst is metallic silver while that in the second stage is of the metal oxide type. The first-stage catalyst actually discussed is silver gauze, although the teaching of the patent is not limited to this. The thrust of Payne is that a more complete over-all reaction is obtained if the metallic silver first stage is followed by use of the oxide catalyst (which employs more oxygen but which also brings about a more nearly complete conversion than does silver.)
U.S. Pat. No. 2,908,715 (to Eguchi et al.) discloses a single-stage oxidation over silver catalyst, with space velocity being discussed as a factor which can be manipulated to cope with the problem of localized overheating of the catalyst.
U.S. Pat. No. 2,504,402 (to Field) discloses a multi-stage catalytic oxidation of methanol. Reactants are cooled between stages, and, according to the patentee, any "well-known catalyst" can be employed although oxide catalysts are exemplified. Field specifically discloses the introduction of methanol between the reaction stages, (as distinguished from Northeimer, who adds air but not methanol), but Field also points out that oxygen can be introduced between the stages if desired. He also discloses the use of a "clean-up" catalyst in the last stage to oxidize all of the methanol down to a fraction of a percent. U.S. Pat. Nos. 3,415,866; 3,640,900; and 3,987,107 all disclose adding methanol between a silver first stage catalyst and an oxide-type second stage to increase reaction productivity.
U.S. Pat. No. 3,174,911 (to Webb) discloses a single-stage oxidation using silver catalyst. Finally, Hedley et al. in "The Industrial Chemist," July, 1952, pages 311 to 316, describe formaldehyde production on the industrial scale by using single-stage oxidation over silver crystals. Hedley et al. refer also to the World War II German technology. To summarize the foregoing, the prior art teaches (a) that silver is an effective catalyst for methanol oxidation and that it can be used in any form having an extended surface with electrolytically-prepared crystals being, however, unusually effective, (b) that silver crystals are particularly useful in the second stage of a two-stage process because they are unusually active catalysts, (c) that in multi-stage catalytic oxidations of methanol to formaldehyde it is possible to introduce supplemental methanol, air, or both between the stages along with, if desired, steam, and that (d) the reaction space velocity and the ratio of oxygen (or air) to methanol in the reactants are significant process control parameters.
Obviously also, of course, the prior art discusses extensively such factors as reaction temperature and, of particular importance, the desirability of reducing the temperature rapidly in the reaction product gases exiting from the catalyst bed. In this matter of rapid temperature reduction, incidentally, co-pending U.S. application Ser. No. 286,235 (by Murphy et al.) discloses an unexpectedly effective technique for enhancing the speed of the temperature reduction following the catalytic reaction by employing a tubular aftercooler the tubes of which are packed with inert inserts such as ceramic balls. This particular technique is a useful adjunct to the present invention which will be described hereinbelow.
It is an object of the present invention to identify certain process parameters the proper manipulation of which results in improvements in the chemical efficiency and conversion obtaining when methanol is catalytically oxidized to formaldehyde in a catalytic oxidation reaction system containing two sequential silver-based catalysts. It is another object to improve reaction performance in a two-stage methanol-oxidation reaction system in which the catalyst in the second stage consists essentially of silver crystals. It is another object to provide a method for improving the performance of the reaction system when each reaction stage employs as catalyst a bed of silver crystals. It is a broad object to provide a control scheme employing as interacting control parameters the formaldehyde:methanol ratio in the second-stage feed mixture and the specific reaction rate in the second stage of the reaction.
Other objects will be apparent from the following detailed specification and claims.